The present invention relates to distributed feedback lasers, and more particularly, to a surface-emitting distributed feedback (SEDFB) laser having curved gratings and a holographic method for fabricating such gratings that is consistent with batch processing of lasers.
The prior art for which the present invention is an improvement is a broad area surface-emitting distributed feedback (SEDFB) laser with chirped or straight gratings. Surface-emitting distributed feedback laser with chirped gratings are described in U.S. Pat. No. 5,241,556, issued to Macomber et al. entitled "Chirped Grating Surface-Emitting Distributed feedback Lasers", and U.S. Pat. No. 5,238,531 issued to Macomber et al. entitled "Apparatus and Method for Fabricating a Chirped Grating in a Surface-Emitting Distributed Feedback Semiconductor Laser Diode Device", both of which are assigned to the assignee of the present invention. Surface-emitting distributed feedback laser with straight gratings are described in "Surface emitting distributed feedback semiconductor laser", by S. H. Macomber et al., Appl. Phys. Lett., vol. 51, pp. 472-474, 1987,"AlGaAs surface emitting distributed feedback semiconductor laser", by S. H. Macomber et al., Proc. SPIE, vol. 893, pp. 188-194, 1988, "Two-dimensional surface emitting distributed feedback laser arrays", IEEE Photon. Lett. vol. 1, pp. 202-204, 1989, by J. S. Mott et al., "Analysis of grating surface emitting lasers", IEEE J. Quant. Electron., vol. 26, pp. 456-465 (1990), by R. J. Noll et al., "Non-linear analysis of surface emitting lasers distributed feedback lasers", IEEE J. Quant. Electron., vol. 26, pp. 2065-2074, 1990, by S. H. Macomber et al., and Recent developments in surface-emitting distributed feedback arrays", Proc. SPIE, vol. 1219, pp. 228-232, 1990, by S. H. Macomber et al.
Although the prior art has demonstrated high power, high efficiency, and good longitudinal beam quality, the lateral beam quality is generally very poor. This problem has limited the usable range in imaging laser radar systems, limited the spot size and depth of focus in applications requiring focusing, and limited the minimum core size of fiber optics into which the laser can be coupled.
The maximum achievable power from a semiconductor laser can be increased by increasing the width of the stripe. However, it has long been known that the beam quality of wide stripe semiconductor lasers is usually many times the diffraction limit. This is described in "A GaAsAl.sub.x Ga.sub.1-x As double-heterostructure planar stripe laser", H. Yonezu et al., in Japan. J. Appl. Phys., vol. 12., pp. 1585-1592, 1973, for example. This problem is caused by self-induced waveguiding that arises from a combination of spatial hole burning and index antiguiding (i.e., the index of refraction of the medium tends to decrease when the local carrier density increases) forming self-guiding filaments. This is described in "Observation of self-focusing in stripe geometry semiconductor lasers and the development of a comprehensive model of their operation", by P. A. Kirby et al., IEEE J. Quant. Electron., vol. QE-13, pp. 705-719, 1977. An initially flat wavefront propagating along a uniform wide stripe tends to break up into self-perpetuating filaments that lead to poor beam quality. This is described in "Spatial evolution of filaments in broad area laser amplifiers", Appl. Phys. Lett., by R. J. Lang et al., vol. 62, pp. 1209-1211, 1993. This problem worsens as the drive current increases, usually with a progressively more aberrated output beam. In comparison, an expanding wavefront should be much less susceptible to this problem.
Unstable resonators have been used with a variety of high power lasers. They produce a high degree of lateral mode selectivity with a mode that fills a large gain region and are relatively insensitive to intracavity index aberrations. This is described in "Unstable optical resonators", by A. E. Siegman, in Appl. Opt., vol. 13, pp. 353-367, 1974. These characteristics combined with curved (expanding) internal wavefronts that can suppress filamentation makes the unstable resonator approach well-suited to the problem of lateral mode control in broad area semiconductor lasers. Unstable resonator Fabry-Perot devices have demonstrated good lateral beam quality. This is described in "High power, nearly diffraction limited output from a semiconductor laser with an unstable resonator", by M. L. Tilton et al., IEEE J. Quant. Electron., vol. QE-27, pp. 2098-2108, 1991, and "Fabrication of unstable resonator diode lasers", by C. Largent et al., Proc. SPIE, vol. 1418, pp. 40-45, 1991. However, fabrication of curved mirrors with required surface smoothness has been problematic.
Presently copending patent application Ser. No. 07/974,775, filed Nov. 12, 1992, entitled "Curved Grating Surface-Emitting Distributed Feedback Semiconductor Laser", assigned to the assignee of the present invention, generally describes curved grating SEDFB lasers. However, this invention only relates to a constant radius grating shape as an example. It was not clear at that time what the optimum shape would be since analysis was not yet available. In the present invention, a curved grating approach is described for achieving lateral mode control in wide stripe SEDFB lasers that is analogous to an unstable resonator. Furthermore, in a second copending patent application Ser. No. 07/975,303, filed Nov. 12, 1992, entitled "Apparatus and Method of Fabricating a Curved Grating in a Surface-Emitting Distributed Feedback Semiconductor Laser Diode Device", also assigned to the assignee of the present invention, a method is described for fabricating a constant radius grating using the Talbot effect but not the more general types of curved gratings described herein. U.S. Pat. No. 5,307,183 and U.S. Pat. No. 5,345,466 are incorporated herein.
Therefore, it is an objective of the present invention to provide for a surface-emitting distributed feedback (SEDFB) laser having curved gratings that overcomes the problems associated with conventional surface-emitting distributed feedback lasers, and to provide for a holographic method for fabricating such gratings that is consistent with batch processing of lasers